You want repossessed carts why? Wouldn't just some other second hand store work ok or a private seller?
If you meant bootlegs though, best options really art to go through aliexpress with a high rated seller, or just get a flash kit, because the n64 has a really fairly priced premium one called SUMMERCART 64 which is remarkably good, better than the grossly expensive (slightly less capable too) krikzz everdrive.
The batteries in old games have lasted for decades and still work, it's a roll of the dice around how they were treated in what environment, but changing them out is super simple even if you have barely any clue how to even use a soldering iron.
